Monday, January 21, 2013

In early spring 2011, just as I was
finishing up my master’s degree at Montana
State, I received an email from Tom
Deméré, the paleontology curator of the San Diego Natural History Museum,
inviting Morgan and I to study a new fossil of Pelagiarctos from the
“Topanga” Formation. Fortunately, I would get a chance to examine it closely in
person soon afterward – in June, I would be attending and presenting my
master’s taphonomy research at the 6th triennial conference on
secondary adaptations of tetrapods to life in the water (usually abbreviated
SATLW or simply referred to as the aquatic tetrapods conference), which was
being hosted by Tom Deméré and Annalisa Berta at San Diego State University and
the museum.

Although I had successfully
delivered my master’s defense presentation and graduated without a hitch a
month and a half prior, I was still nervous to give my presentation because it
was in front of a totally different audience – technically, the conference was
about secondary adaptations, and I was giving a talk on taphonomy. However, I
tooled it towards what we can reasonably infer from the marine vertebrate
fossil record, including about exactly how aquatic organisms were based on
their preservation – which, I concluded at the time was not much. The talk was
also fairly long; although I had 18 minutes to speak, which is fairly long, I
had not had the time to shorten it. 36 hours before driving down I-5, I was on
the beach at Bolinas prospecting with Dick Hilton when I got a funny phone
message from Tom ‘asking’ me if it would be okay to move my talk to the first
day; so I said my goodbyes to Dick and raced home down Highway 1 so I could
spend a day and a half polishing the presentation off. And then worried half
the drive down I-5 that I didn’t shorten it enough.

A comparison of the new Topanga Formation specimen (A) and the holotype (B) of Pelagiarctos.

The talk went without a hitch, and
later in the conference Morgan and I were able to sequester a few hours in the
SDNHM type room to examine the new specimen of Pelagiarctos. It
consisted of a fragmentary pair of mandibles, with the left mandible being
nearly complete and having much of its dentition (missing only a premolar, the
two molars, and an incisor). Unlike the type specimen from Sharktooth Hill
(which Morgan and I got a chance to examine in person at LACM in January 2012),
these mandibles were not fused together at the symphysis (intermandibular
joint). Symphyseal fusion is not common in modern pinnipeds, where it is
restricted to the modern walrus. I’ve also seen pathologic (diseased) mandibles
of modern otariids where, due to some bone disease, the symphysis has fused
along with a large mass of bone at the chin, accompanied by incisor and canine
loss.

The left mandible of the Topanga Formation specimen of Pelagiarctos. From Boessenecker and Churchill (2013).

The teeth present in the new
specimen confirm that the large teeth referred to Pelagiarctos thomasi
by Barnes (1988) were correctly referred. It’s not so surprising, since you
could predict the mandible shape from the teeth: they are like giant versions
of Neotherium teeth, and the mandible is like a giant Neotherium
jaw. I never really doubted Barnes’ identifications – but it was nice to confirm
them. The mandible of this new specimen is damned huge – it’s wide, deep, with
a short toothrow. The canines are robust, again with grooves on the lateral and
medial surfaces, giving the canine a figure-eight shape in cross section. The
premolars are large, primitively retaining what’s called the metaconid cusp;
most modern pinnipeds have teeth that are unicuspid (single cusp, usually
conical), with small anterior and posterior accessory cusps in otariids. The
main cusp (on lower postcanine teeth) is called the protoconid. The anterior
and posterior cusps are the remnants of the paraconid (anterior) and hypoconid
(posterior) cusps. The metaconid cusp is a fourth cusp which is present in many
primitive pinnipeds, such as the early enaliarctines, as well as Proneotherium,
Neotherium, and Pelagiarctos. The metaconid is located just
behind the principal cusp (protoconid). Many modern phocids primitively retain
all four cusps – the harbor seal is an excellent example. The crabeater seal
additionally bears a number of extra “neomorphic” (=new or novel structure)
cusps on the posterior tooth crowns, which are used for filter feeding.

The dentition of the Topanga Formation specimen. From Boessenecker and Churchill (2013).

More details of the dentition of the Topanga Formation Pelagiarctos, from Boessenecker and Churchill (2013); I had a fun time drawing the medial view of those teeth.

In addition to having these
primitive features, a couple of new features not seen in earlier walruses are
present: a lingual cingulum with small little “crenulations” forming a sawtooth
type pattern, and the presence of a labial cingulum. A cingulum is a thickened
ridge of enamel at the base of a tooth crown. Pelagiarctos is the only
walrus with a labial (cheek side of the tooth) cingulum, and only one other
walrus has a crenulated lingual (tongue side of the tooth) cingulum – the late
Miocene walrus Imagotaria downsi. At this point the uninitiated reader
might ask ‘what exactly makes this thing a walrus?’ The truth is, for the earliest
known walruses, the only synapomorphies allowing identification as a member of
the Odobenidae (walrus family) are skull features. Many of the features of the
known specimens of Pelagiarctos appear in some sea lions – such as a
mandible that is deepest near the canines. Although the fossils don’t have any
specific features that are diagnostic at the family level – several features of
the dentition are only found in early diverging “imagotariine” walruses. The
Imagotariinae was a subfamily named by Ed Mitchell and used extensively in
various papers by Barnes, but as pointed out by several studies over the past
two decades it is a paraphyletic assemblage of primitive walruses.
Nevertheless, it is a useful vernacular term; imagotariines are sea lion-like with
primitive dentitions, and ranged in size from harbor seal size (Proneotherium)
to elephant seal size (Pontolis magnus).

Comparison of walrus mandibles, including the Topanga Fm. Pelagiarctos specimen, Imagotaria downsi from the late Miocene Santa Margarita Sandstone of Santa Cruz County, Proneotherium repenningi from the early middle Miocene Astoria Formation of Lincoln County, Oregon, and Pontolis magnus from the late Miocene Empire Formation of Coos County, Oregon. From Boessenecker and Churchill (2013).

Because our new specimen was more
complete than the holotype, we were able to include Pelagiarctos within
a phylogenetic analysis for the first time. Previous analyses did not use many
mandibular characters, so at first we constructed a matrix which focused on
mandibular and dental characters, and only used pinniped species known by lower
jaws (i.e. we didn’t include some species for which jaws were unknown). This
meant we didn’t initially include the early walruses Prototaria and Pseudotaria
from Japan.
We originally did this because we felt we’d get more accurate results than if
we included Pelagiarctos in an analysis where it couldn’t be coded for
any cranial characters – it was a reasonable hunch at first. One of our
reviewers suggested we use a more comprehensive dataset, so we merged our data
set with that of Naoki Kohno’s (2006) analysis for his Pseudotaria muramotoi
paper. We ended up with fantastic results, and better support for some of the
relationships.

Cladograms from Deméré (1994), Kohno (2006), and our new study showing the varying position of Pontolis (underlined in red).

Most of the relationships in our
analysis are consistent with previous studies like Deméré (1994), Deméré and
Berta (2001), and Kohno (2006), with one exception. In Deméré (1994), Pontolis
magnus grouped as a dusignathine walrus, and closely related to Dusignathus
itself. In Kohno (2006), Pontolis instead formed a sister taxon
relationship with Imagotaria. The Imagotaria-Pontolis clade is only one
node below the dusignathines, so admittedly it is not a far distance. In our
analysis, however, Pelagiarctos formed a sister taxon relationship with Imagotaria
instead, based on two features: grooved canines, and a crenulated cingulum.
Neither of these features are present in Pontolis. Instead, Pontolis
plotted out as the last diverging “imagotariine” and the sister taxon to the
dusignathine + odobenine clade – in other words, intermediate between the
phylogenetic hypothesis of Deméré (1994) and Kohno (2006). It’s sort of a
compromise between the two, in a way. Obviously, there are more cranial
characters that need to be explored and new walruses to describe, so there is
clearly further scope for a more comprehensive study of walrus phylogenetics,
which is in the early planning stages.

Next up: the dramatic conclusion to
this series on the new publication, focusing on the feeding ecology of Pelagiarctos, and the life restoration.

Boessenecker, R.W. and M. Churchill. 2013. A reevaluation of the morphology, paleoecology, and phylogenetic relationships of the enigmatic walrus Pelagiarctos. PLoS One 8(1) e54311. doi:10.1371/journal.pone.0054311.

Deméré TA (1994) The family Odobenidae: a phylogenetic analysis of living and
fossil forms. In: Berta A, Deméré TA, editors. Contributions in Marine Mammal
Paleontology honoring Frank C Whitmore, Jr: Proceedings of the San Diego
Society of Natural History. 99–123.

Last thursday my new study which I collaborated with Morgan Churchill
(University of Wyoming) on was published in PLOS One, regarding new
fossil material of Pelagiarctos from the "Topanga" Formation of Orange County, California. There has been quite a bit of buzz about it, and it's gotten a surprising amount of media attention. To summarize it in one sentence - we describe the new material, reanalyze the paleoecological hypothesis of Barnes (1988), concluded it was not a specialized macrophagous predator, and conducted a phylogenetic analysis of the Odobenidae (walruses).

Brian Switek was kind enough to cover it on Laelaps, which you can see here. Also, there is an author spotlight on the PLOS EveryONE blog, viewable here.

Part of the new specimen of Pelagiarctos, which Tom Deméré(San Diego Natural History Museum) invited us to study.

Life restoration of Pelagiarctos, which I did last fall in my spare time. More on how I put this together at a later point.

Tuesday, January 15, 2013

As a teaser for a forthcoming paper by Morgan Churchill and
myself, I thought I’d introduce a (short) new series of posts (fewer than the
last series, I promise). As the publication is not out quite yet, I
thought I could at least give an introduction to the extinct “killer” walrus
from the Sharktooth Hill Bonebed.

The Sharktooth Hill bonebed in Kern
County, California is a
widespread horizon within the Round Mountain Silt member of the Temblor
Formation. It’s exposed near Bakersfield, California,
and is middle Miocene in age. It’s approximately 10-50 cm thick, generally
lacks calcareous invertebrate fossils, but is extraordinarily rich in teeth and
bones of sharks, bony fish, birds, sea turtles, pinnipeds, dolphins, sperm
whales, baleen whales, and occasionally sea cows, desmostylians, and
terrestrial mammals. I visited Sharktooth Hill several times as a high school
student, trying to find “local” vertebrate fossils – digging well through the
night in the trenches with tiki torches and a headlamp. At many localities
frequented by amateur fossil collectors, the bonebed is exposed on a hillside
and a large linear scar follows the position of the bonebed, dug out by
collectors removing overburden to get to the fossil layer. Amateur fossil collectors have done so much digging that a trench reminiscent of World War 1 battlefields encircles many hills in the region where the bonebed is exposed. Although some collectors will spend days at a time digging through overburden - admittedly backbreaking work - some decide to risk it and tunnel into the trench to get at the bonebed. Some collectors have paid for this tactic with their lives: on my first visit in 2002, a cross was placed at one of the localities where a collector had tunneled in about ten feet and was killed when the hillside slumped down onto him; it took the authorities several days to dig out his body. The rest of the
Round Mountain Silt is mostly barren with respect to vertebrate fossils, not only explaining the attention given by collectors to the bonebed itself –
but also suggesting a “unique” environment temporarily persisted in order to concentrate
vertebrate remains. A number of strange biologic explanations have been
offered, including red tides, extensive shark predation, and even a marine
mammal calving ground. Several authors have quite rightly scrutinized these
biologic explanations, and have suggested sedimentologic processes as a cause
(Mitchell, 1966; Prothero et al., 2008; Pyenson et al., 2009). These studies
have specifically suggested that a depositional hiatus (slowdown in the
accumulation rate of sediment) permitted marine vertebrate remains to be
concentrated on the seafloor. I have some minor taphonomic reservations, but
those are best discussed another day.

One of the Sharktooth Hill localities, wife for scale.

According to Barnes (1976), the Sharktooth Hill bonebed is
the most extensively studied and richest marine mammal locality in the eastern
North Pacific; a faunal list compiled by amateur collectors can be viewed here,
and it includes roughly 140 vertebrate taxa. Some of the species on the list
are not yet described or published (“Neotherium ernsti”, for example)
and other taxa are based on old identifications and may not be borne out in the
long run (aff. Herpetocetus). Regardless of issues pertaining to the
taxonomic identity of some fossil vertebrates, the ballpark number is probably
accurate. It’s also fairly spectacular: I recently tallied up fossil
vertebrates from the Purisima Formation, and there are roughly 70 taxa present
– still impressive as hell, but not quite as gargantuan as Sharktooth Hill.
Depending upon whose publication you look at, there are anywhere from seven
(Barnes, 1972; Barnes and Hirota, 1995) to four pinnipeds present (Deméré et
al., 2003). Papers by L.G. Barnes and colleagues list several desmatophocids,
including Allodesmus gracilis, Allodesmus kelloggi, Allodesmus
kernensis, Desmatophocine B, and Desmatophocine C in addition to the
imagotariine walruses Neotherium mirum and Pelagiarctos thomasi.
According to Deméré et al. (2003), only four taxa are present – Allodesmus
kernensis (with A. kelloggi and A. gracilis subsumed as
junior synonyms), an indeterminate desmatophocid (Desmatophocine B), and the
two walruses. While it’s nowhere near as diverse as the cetacean assemblage
from the same locality, it’s fairly comparable with other fossil pinniped
assemblages from the eastern North Pacific.

The skeleton of Allodesmus kelloggi as exposed in the field. From Mitchell (1966).

In 1980,
future chief preparator of the Los Angeles County Museum of Natural History (LACM)
discovered a curious chunk of bone with teeth at Sharktooth Hill. Several years
later, he brought it in to LACM and showed it to Dr. L. G. Barnes (colloquially
known as ‘Larry’ within the field), and insisted that it was the piece of a
snout of some extinct mammal – it even had two small holes which look like
nostrils to the uninitiated. Barnes kindly pointed out that those were mental
foramina on the “chin” end of a very large jawbone of a pinniped. Larry and
Howell enthusiastically recalled this whole story for Morgan Churchill and I
when we sat at the very same table last January, thirty or so years later
(Larry Barnes has an incredible, near photographic and certainly encyclopedic
memory of marine mammal fossil specimens). Howell Thomas donated the fossil for
study, and within a few years was hired as the Chief Preparator, and Barnes
began to study the specimen. At the time, the marine mammal assemblage was
already enormous, and the pinniped assemblage well documented by hundreds of
specimens. Most of the fossils could be assigned to the large seal-like Allodesmus,
although a single jaw described by Barnes (1972) as “Desmatophocine B” didn’t
appear to be referable. “Desmatophocine B” was probably similar to Allodesmus,
which has a long narrow skull, enormous eye sockets, single-rooted teeth, and a
relatively large body. Furthermore, we know Allodesmus retained the
ability to rotate its hindflippers forward for sea-lion like terrestrial
locomotion, and it was probably a sea-lion like underwater “flyer”. Numerous
small pinniped elements appeared to be similar to a handful of elements
described by Remington Kellogg (1931) as Neotherium mirum.

Skulls of Allodesmus (left) and Neotherium (right) roughly to scale. From

Barnes and Hirota (1995) and Kohno et al. (1995).

Neotherium was an enigma for
over 60 years, and it wasn’t until more complete remains of the early walrus Imagotaria
downsi were recovered from the Santa Margarita Sandstone near Santa
Cruz, California, that Neotherium
began to make sense. Imagotaria was a sea lion-like walrus that lived
about 9-12 million years ago – a bit younger than the 15-16 Ma Sharktooth Hill
Bonebed – and by the close of the 1970’s was known by a number of well
preserved skulls and partial skeletons from Santa Cruz County. Fossils of Neotherium,
although never as common as Allodesmus, continued to trickle in from the
bonebed and were referred to Neotherium piecemeal, one or two bones at a
time by Mitchell (1961), Mitchell and Tedford (1972) and Repenning and Tedford
(1977). By the 1980’s, Barnes had amassed a collection of nearly every skeletal
element of Neotherium, identifiable as miniature and slightly more
primitive versions of that found in Imagotaria – including partial
skulls and several mandibles (eventually a complete skull was published by
Kohno et al. 1995). Barnes has been for many years working on a monograph on Neotherium
– I’m looking forward to seeing it published.

The holotype of Pelagiarctos thomasi. From Barnes (1988).

Howell Thomas’ mystery jawbone
appeared more similar to Neotherium relative to Allodesmus, with
the exception of its comparably gigantic size as well as having a fused
intermandibular joint (mandibular symphysis) and deep grooves on the sides of
the canines. Eventually, several isolated teeth that were similar to Neotherium,
but several times larger in size – were discovered from the bonebed. Some of
these teeth even fit right in to the tooth sockets in the mandible fragment.
Barnes published the fossils in 1988 and described them as Pelagiarctos
thomasi, the species name honoring Howell Thomas. The genus name Pelagiarctos
refers to the primitive dental anatomy, as ‘arctos’ refers to bears, the traditional
sister taxon of pinnipeds (the root arctos is frequently used in pinniped genus
names – Arctocephalus, Phocarctos, Hydrarctos, Pteronarctos,
etc.), as well as the inferred pelagic ecology of the animal.

The isolated teeth referred to Pelagiarctos by Barnes (1988).

Several aspects of the anatomy of Pelagiarctos,
although based on scant material, suggested a different approach to feeding in
this fossil walrus relative to other Sharktooth Hill Pinnipeds. The teeth of Pelagiarctos
were huge – very robust canines, and postcanine teeth with multiple large
cusps and sharp crests. He likened the premolars and molars to those of modern
hyenas and extinct borophagine dogs, two groups which (by observation or inference)
crack and ingest bones, suggesting that Pelagiarctos
had dental adaptations for large bite forces related to feeding on large
prey items. Furthermore, the robust mandible and fused symphysis further
suggested high bite forces. Barnes (1988) additionally noted that Pelagiarctos
is very large and numerically rare in the Sharktooth Hill Bonebed – only known by
five teeth and a mandible fragment at the time of his study, as opposed to the
hundreds of specimens known of other pinnipeds such as Allodesmus and Neotherium.
This suggested to Barnes that Pelagiarctos
was rare in California waters during the middle Miocene, further supporting his
hypothesis that it was an apex predator (apex predators at the top of the food
chain can never be very abundant because they rely on a constant stock of
abundant prey items). Barnes further postulated that the type specimen was a
male, as it had proportionally large canines; modern and fossil pinnipeds are
sexually dimorphic, including early walruses like Neotherium, Imagotaria,
and Proneotherium. One of the canines
in the holotype is broken and polished down, suggesting the tooth had been
broken and worn down after continued use in life – damage which Barnes
attributed to male combat, which occasionally results in such damage in modern
pinnipeds. Furthermore, Barnes identified some of the fossil teeth as males
because they fit right into tooth sockets on the type specimen, and those that
didn't were of similar size.

As a result of these hypotheses,
numerous fanciful reconstructions of Pelagiarctos have been produced by
paleoartists (fanciful depictions can be seen here, here, and here). and Pelagiarctos has achieved the nickname
"killer" walrus by some enthusiasts. But what do we really know about
Pelagiarctos? Stay tuned...

Mitchell
ED, Tedford RH (1972) The Enaliarctinae: a new group of extinct aquatic Carnivora
and a consideration of the origin of the Otariidae. Bulletin of the AmericanMuseum
of Natural History 151: 203-284.

Monday, January 14, 2013

Congratulations are in order to my colleague and friend Rachel
Racicot, a Ph.D. student at Yale (working under Jacques Gauthier), for
getting her master's research published in the Journal of Morphology.
Rachel did her Master's at San Diego State with Annalisa Berta, and was
just finishing up when I visited the San Diego NHM for the first time in
2007. Aside from functional morphology and the endocranial anatomy of
odontocetes (particularly cranial sinuses, brain endocasts, and the
inner ear), Rachel is also interested in fossil porpoises and is
currently researching the strange "half-beaked" porpoise from the San
Diego Formation. Rachel's master's research concerns the pterygoid sinus
morphology of modern porpoises.

Ms. Racicot had no idea that the journal had selected her image to be put on the cover

of the January 13 issue, and she was quite surprised when I congratulated her on it. Pleasantly

surprised, I should say.

Before
I continue, I should briefly introduce phocoenids. The Phocoenidae, or
true porpoises, are a small bodied group of delphinoid cetaceans that
are not terribly diverse (6 species, 3 genera) in comparison to oceanic
dolphins (Delphinidae; ~40 species, ~12 genera). They differ from
delphinids in having short rostra, having symmetrical skulls, large
bumps on the premaxillae just before the bony nares, and have inflated
braincases without large bony crests. Phocoenids are considered to be
paedomorphic -that is, retaining juvenile features into adulthood, thus
explaining A) their inflated, juvenile-like braincases, B) lack of
strong bony crests, C) cranial symmetry, D) short rostra, and E) small
body size. It should be noted that the delphinid Cephalorhynchus is
thought to parallel phocoenid paedomorphosis. Modern phocoenids also
have strange, spatulate teeth which almost resemble the teeth of
nodosaurs and ankylosaurs. Many fossil phocoenids, on the other hand,
have longer rostra, conical teeth, cranial asymmetry, and better
developed cranial crests.

Schematic view of a neonatal harbor porpoise (Phocoena phocoena) skull showing in blue the various parts of the pterygoid sinus. From Racicot and Berta (2013).

The
pterygoid sinus is present in all Neoceti, and even within
basilosaurids. It originates as an outpocket of the eustachian tube (an
air filled cavity present in the middle ear of all mammals - hold your
nose with your fingers and blow, and you'll feel crackling in your
eustachian tubes; they are also what "pop" when changing altitude as the
pressure changes). Parts of the sinus system can be seen externally,
such as the hamular lobe of the pterygoid sinus, which is not completely
encased in bone and is visible in a prepared skull as large cavities
surrounded by thin flanges of bone. The pterygoid sinus system is
elaborated in odontocetes relative to mysticetes. Although known to
exist, the anatomy of the pterygoid sinus system in odontocetes - and
true porpoises (Phocoenidae) in particular - is difficult to assess.
Since they are cavities within a solid object, it's difficult to study
them by any conventional means as they remain hidden in the skull.
Certain aspects of the pterygoid sinuses have, for example, been used in
phylogenetics - in multiple phylogenetic analyses which have included
phocoenids, a cladistic character has been used - presence or absence of
a dorsal extension of the preorbital lobe of the pterygoid sinus
between the maxilla and frontal bone (po on the above diagram). This is a
phocoenid feature, and the bottlenose dolphin lacks this. The
preorbital lobe is well developed in the neonatal specimen (neonate =
newborn individual, rather than a juvenile or subadult), although the
dorsal extension is not as well developed as in the adults (an example
of ontogeny recapitulating phylogeny).

Digital 'endocast' of the right pterygoid sinus (in medial view) from six skulls of Phocoena phocoena; anterior is to the left. Neonate specimen shown in F. The sinus shape looks pretty weird (but then again, so does the rest of a cetacean skull).From Racicot and Berta (2013).

So,
what's it for? Previous hypotheses for the function of the sinus
includes A) an acoustic barrier to reflect sounds produced during
echolocation forward through the melon, B) an acoustic barrier between
sound producing and sound receiving structures (e.g. nasal region and
petrotympanic complex, respectively), and C) to acoustically isolate the
petrotympanic complex from sounds produced during echolocation (which,
admittedly, sounds similar to B). A fourth hypothesis posits that the
pterygoid sinus serves as a means to regulate pressure around the middle
ear during diving.

The paired sinuses (right and left) of Phocoena phocoena specimens in anterior view. Note that there is right-left asymmetry in each specimen.From Racicot and Berta (2013).

To
test the sound reflecting ability of the sinus, Racicot and Berta
calculated the minimum thickness necessary to reflect sound at the
typical highest frequency sounds produced by Phocoena phocoena (~150
kHz). Many aspects of the cetacean skull make sense in the light of
acoustic impedence - sound waves tend to bounce off of objects or
features where there is a stark change in density. For example, echoes
in air are sound waves bouncing off a solid surface. In water where the
medium is much denser, sound not only travels faster, but flesh and bone
are so similar in density that sounds travel through the vertebrate
body rather than bouncing off of it - making things like external ears
(which take advantage of sound waves bouncing due to acoustic impedence,
and funnels sound in) useless. So, within a skull, a wall of air within
a sinus is different enough in density to reflect sound, analogous to a
solid object in air.

They calculated that the
preorbital lobe would need to be 2.5mm thick at a minimum, which is less
than what they observed in phocoenid sinuses - indicating they would
function well at reflecting sounds. As for the asymmetry of the sinuses,
they remarked that this could be explained by the fact that experiments
have determined that porpoises produce sounds in an asymmetric fashion,
preferring to use one nasal passage over the other, potentially
explaining why the sinuses are asymmetrical.

Wild
speculation time: it's also possible that aysmmetrical sinuses may be a
vestige of cranial asymmetry. Fossils show that the earliest phocoenids
had asymmetrical skulls in a similar fashion to delphinids; perhaps this
is an example of phylogenetic inertia - the external skull changed at a
faster pace than the sinuses, reaching symmetry first. However,
paedomorphosis typically progresses by delaying adult morphology later
and later during ontogeny, and retaining juvenile features longer and
longer instead. In other words, paedomorphosis would suggest that
asymmetry was once an adult feature which at some point was lost because
juvenile symmetry prevailed - which doesn't totally jive with
asymmetrical sinuses being retained, unless the two are decoupled
somehow, progressing along different ontogenetic trajectories. Or, is
asymmetry so ingrained within odontocetes that it's a juvenile feature
in phocoenids, with symmetry really being secondarily gained via
hypermorphosis, with asymmetry being pushed earlier on in ontogeny?
Interesting questions, but they remain unanswered. We need more fossils
and further studies of modern phocoenid cranial anatomy.

Another last thought - it's interesting to note that phocoenids are considered paedomorphic, but have relatively extensive pterygoid sinuses. The primitive condition among Neoceti, of course, is possessing less well developed sinuses (pterygoid sinuses
in Neoceti and Basilosauridae are acquired stepwise in a piecemeal
fashion). In other words - sinus development is not showing a
paedomorphic trend - in fact, it's showing the opposite trend - it's a
peramorphic feature, probably undergoing something like hypermorphosis
(development is postponed and extended later into ontogeny) or
acceleration (faster development of a feature during ontogeny). Perhaps
hypermorphosis is not likely, given the short period it takes for
phocoenids to mature.

Friday, January 11, 2013

Perhaps it was a slow news day in Dunedin since there are no rascal scarfie students burning couches in the streets (it's summer break here, and relatively hot today), but we were interviewed and photographed for the Otago Daily Times yesterday. I don't have permission to include a copy of the photo here, but you can see it here on the Otago Daily Times webpage. Unfortunately fellow marine mammal students Cheng-Hsiu Tsai (currently conducting research in Japan) and Moyna Mueller weren't around.

Wednesday, January 9, 2013

As covered in a previous post, Kiel and colleagues (2011) recently reported on Osedax traces in Oligocene marine bird bones (Plotopteridae) from Washington, implying that by the late Paleogene Osedax was
adapted towards using bones of many types of marine vertebrates (birds
and cetaceans). This has direct implications for the divergence time of Osedax -
two dates have been determined by molecular divergence dating: Eocene
(coinciding with the radiation of pelagic cetaceans), or Cretaceous (a
period with numerous large marine reptiles). Kiel et al. indicated that
medium sized birds spanned the K/Pg interval and were some of the only
marine tetrapods with available bony substrate during certain areas in
this time interval. Fish bone, of course, is always going to be more
plentiful in the marine record - but fish bones are generally small, and
at the time of publication, Rouse et al. (2011) had not yet published
their experimental colonization of fish bones by Osedax. Later
that year, Higgs et al. (2011) indicated that the bimodal size of
boreholes and lack of distinct individual cavities could suggest that
Kiel et al. (2011) had not really reported Osedax colonizing
bone, but that the traces on bird bones may actually be sponge borings -
sponges form a trace called Entobia, which looks (on typical substrate
like mollusk shells) like a bunch of tiny pinholes; the sponge inhabits
the cavity, using the numerous tiny pinholes for inhalent papillae, and a
few larger pinholes for the exhalent papillae.* Additionally, the
borehole density reported by Kiel et al. (2011) was far denser than
reported on experimentally colonized whale bones, which Higgs et al.
(2011) further identified as evidence that the Oligocene bird bones were
not in fact Osedax-bored, but sponge-bored.

Close up shot of the bone surface, showing the two sizes of pinholes in the Oligocene plotopterid. Are they the sponge trace Entobia, or the Osedax trace Osspecus? From Kiel et al. (2011).

Steffen Kiel and colleagues followed up their previous discoveries with a new paper in Paläontologische Zeitschrift, reporting in more Oligocene marine vertebrates bored by Osedax: fish bones and whale teeth. Altogether, it's not all too surprising: we have borings in a wide array of modern and fossil critters already. We already have modern fish bones - the really cool thing, in my opinion, is that Osedax will consume teeth. At the moment, I'm wrapping up a large manuscript which does include a little blurb and a figure showing possible Osedax borings in a dusignathine walrus tooth - which means I've got to add a new reference to my paper. But that's a story for another day.

The
fossils hail from various Oligocene deep water rock units which are
already known to produce whale fall faunas, cold seep assemblages, and
wood falls. These include the Makah and Lincoln Creek Formations of the
northern coast of the Olympic peninsula, as well as the (also) Pysht
Formation just across the river from beautiful Astoria, Oregon. Coauthor
Jim Goedert and his wife Gail have prospected the rugged coastline of
the Pacific Northwest for decades, finding marine mammals, sharks, and
marine birds. He's described Paleogene pelagornithids from the area, and
named a plotopterid (Phocavis). The toothed mysticete Chonecetus goedertorum
from Washington was named for them (they collected the holotype in
1984), and Gail also found what would later become the holotype of Pteronarctos goedertae in Oregon.

Arguably
the most surprising finding of Kiel et al's new study is the pervasive
boring of whale teeth. These teeth were all found in a partial, highly
corroded mandible; the authors did not specify what kind of corrosion,
but it was in all likelihood bioerosion (and Osedax related at
that). They didn't identify what kind of mysticete the teeth belonged to
- published aetiocetids from the Oligocene of Oregon and Washington
have simpler teeth, but I have seen aetiocetid teeth in USNM collections
similar to these. In several specimens, the crown was heavily bioeroded
by Osedax, and the root just below the crown was as well. They
argued that the loss of the crown in some cases was caused by scavenging
invertebrates (possibly crustaceans) accidentally damaging the root
while trying to eat the Osedax worms. Similar crustacean-mediated
destruction of bone has been observed in modern whale falls. (As a
total aside, it's mid summer here in New Zealand, and both my wife and I
got pinched on the toes by shore crabs while in the water.)

CT images of the Osedax borings within the whale teeth, with individual borings shown in yellow in 3D below. From Kiel et al. (2012).

Kiel
et al. (2012) also took the opportunity to respond to some of the
comments by Higgs and colleagues about the bird bone traces. Kiel et al.
pointed out that in numerous modern specimens, they observed
comparatively dense Osedax borehole clustering, so extreme
borehole density does not invalidate the their identification. Kiel et
al. (2012) also point out that there were mollusk shells present along
with the bird bones, and the shells were not bored; shells are the
typical substrate for such boring sponges. As a taphonomist, I should
note that this argument doesn't necessarily hold sway: most marine
assemblages are time averaged, and because vertebrate and mollusk
remains are of different chemistry, size, and have different soft tissue
anatomy and production rates, they are subjected to different
taphonomic pathways. In other words, it is certainly possible that the
period of modification was different for the bones and shells. To make
that more clear: the whale fossil could have been sitting on the
seafloor exposed for a long period of time, allowing Osedax to
colonize; towards the end of the pause in sedimentation, some mollusk
shells are washed in and buried too quickly for the shells to have also
been colonized. Or, the mollusks could have been burrowing taxa. Back on
topic: Kiel et al. (2012) concluded that these fossils demonstrate that
Osedax has been a generalist bone-consumer for over thirty million years, which strikes another blow to the "Osedax as a cetacean bone specialist" hypothesis.*I'm
no expert on Poriferan anatomy, but I can only assume that the papillae
are homologous to the large openings through which water is pumped in
and out.Don't forget to see the other posts in this series:Bone-eating zombie worms, part 3: Osedax consume more than cetacean bones

Sunday, January 6, 2013

I realized that my photos from Charleston did not really include much in the way of candid photos, or photos of us (you know, people).
My camera has a narrow field of view and is ill suited for getting 'action' photos. Fortunately, my colleague Tatsuro Ando has graciously allowed me to
post some photos he took of the gang and exhibits while in Charleston.
Thanks, Tatsuro!

Another view of King Street in Charleston.

Ewan and I enjoying some beer after a long drive from North Carolina. This was my first time eating real barbecue after starting my Ph.D. program, and damn was it good.

Ewan and I looking at the model of the Hunley outside the Charleston Museum.

The pier at Folly Beach, South Carolina - we ate dinner at a restaurant at the base of the pier.

The Sanders' treated us to dinner at a great seafood place in Folly Beach (south of downtown Charleston). I tried shrimp n' grits for the first time - it was delicious. From left to right, clockwise: Rhonda Sanders, Al Sanders, Ewan, Myself, and Eric Ekdale.

Al and Ewan on the Sanders' screened in porch, overlooking the South Carolina 'forest' (better termed jungle, in my opinion).

The view out into the forest; a few minutes later, a raccoon came climbing by. It was a welcome sight, although it is pretty neat seeing brushy tailed possums here in Dunedin.

After a wonderful seafood dinner, discussions of cetacean evolution, key lime pie, and genuine southern hospitality - and we were ready to take a cab back into town. (From left: Eric, yours truly, Rhonda, Al, and Ewan).

A skull of Schizodelphis from the Calvert Cliffs, on display at the College of Charleston.

A cacophony of mosasaurs on display at the College.

Ewan inspecting some Carcharocles megalodon teeth, unaware of how funny this ended up looking.

Basilosaurid teeth from South Carolina (College of Charleston).

Desmostylian teeth from the middle Miocene Temblor Formation of California (College of Charleston).

To examine fine details of mysticete palates, sometimes you have to get in pretty close.

Someone else (Ewan I suspect) snapped a pinup-esque picture of me doing this, which has hopefully

been deleted forever.

A mounted cast of Pteranodon at the College of Charleston.

A Platecarpus (mosasaur) skull and cervical vertebrae mounted at theCollege of Charleston.

The gang's all here: Tatsuro, myself, Ewan, Al, and Eric in Charleston Museum collections. Thanks, Al, for a great time (and good food!).

Saturday, January 5, 2013

We're finally at the conclusion of this long set of posts about my month-long trip back to the US. I've sort of told it in sequence, with the exception of SVP, which was in the middle of the trip, and right before the trip down to Charleston. The SVP meeting was great - my 8th meeting, and the 6th at which I've presented research. I gave my second SVP talk this year - this time, I presented results of my master's thesis research on marine vertebrate fossils from the Purisima Formation near Santa Cruz, California.

I carried out my master's research from 2008-2011 at Montana State, and collected taphonomic data from fossil vertebrates I had collected, as well as specimens from UCMP and the Santa Cruz Museum of Natural History. We have a very poor concept of the taphonomy of marine vertebrates - and sought to clarify some of these issues by studying changes in preservation among different shallow marine depositional settings.

Title page from my SVP talk. The image is taken near Halfmoon Bay, with my wife

Most previous taphonomic studies of marine vertebrates have focused on single skeletons or bonebeds - which admittedly doesn't tell us much about the big picture. I'll spare you the details for now (least of all because it's not published yet), but the research addresses some of these big picture questions and patterns. The talk went off without a hitch, and I was able to meet with quite a few colleagues. The night before the talk, I did benefit from finding a jacuzzi with a few friends in a vain attempt to relax. I'd given the talk before, three other times - at my defense, at the aquatic tetrapods meeting, and again at Fossil Coffee at UCMP - but getting up in front of a huge audience at SVP is something else altogether. Unfortunately, because the talk was on the last day of the conference, the meeting flew by way too quickly - one of the reasons I enjoy poster presentations much more.

SVP is also a fantastic time because it's one of the few times you ever get to see old friends. Many friends of mine I had not seen since my wedding last year; it was a bit difficult waking up the day after SVP and not being able to go find mobs of familiar faces milling about in the hotel lobby. Nevertheless, there is always next SVP.

I'm currently nearing submission of the manuscript version of my master's thesis, something I've been looking forward to for a long time. I try to keep projects moving, and would rather not get too caught up and sit on them; it's only been a year and a half since I graduated and completed my master's, so I suppose I'm doing well. The manuscript has taken a lot of time to modify - I've spent almost as much (or even more) time editing it over the past 6 months than I did writing it in the first place, and it is a far better piece of writing because of it. With this, and the September submission of another monstrous manuscript (also 100+ pages), I've cleared off a sizeable part of my research backlog.

After SVP, and Charleston, I had another week in Washington D.C. I was scheduled to fly out to California on Halloween. I kept hearing things on the news while in Charleston about a hurricane, but didn't really think too much of it - after all, hurricanes only happen on the east coast, right? As a Californian, I've always sort of filtered out news like tornadoes and hurricanes. And then I remembered, like some sort of bizarre moment of existentialist realization - that I was in Washington D.C. of all places, with a gigantic hurricane heading more or less right for us, with landfall in about four or five days. Fortunately, I was staying in D.C. with my childhood friend (and aide for the Senate Committee on Veteran's Affairs) Ben Merkel - who lives in a brownstone north of downtown, on a sloped street far from any drainage, and separated from the street by about 15 feet of steps. I felt pretty safe. Unfortunately, it also meant that I'd have to miss two days of museum work. Well, shit happens, and I was able to come in over the weekend before Sandy hit. We spent the 48 hour period while the city was shut down watching movies, eating and cooking, and drinking a healthy amount of beer. We only lost power for half a second when the lights flickered - once. We had internet the whole time, and I was able to work on some peer reviews and writing up a short manuscript on Herpetocetus. It was actually a pretty fun and productive time. Best of all, I was even able to make it out to the Garber facility for a few hours on my last day in DC to get some much needed photos. The storm had largely bypassed DC - we had some pretty heavy rain and high winds, and they had to close down the DC metro and buses and sandbag a bunch of federal buildings, but all the damage we saw included a few newspaper stands blown into the street. And my flight even left on time!

All in all, the trip was a total success, and I was able to arm myself with all the data and photographs necessary to complete my thesis here in New Zealand. I even had enough time while on the trip to start and finish an entire manuscript (a short one, anyway).

Thursday, January 3, 2013

At the welcome reception at the North Carolina Museum of Natural History, I spoke with my colleague and good friend Jonathan Geisler about a new collection of fossil cetaceans at the College of Charleston, being managed by Mace Brown, shown below. I was pretty excited about it after hearing Jonathan describe it; I was totally blown away when I saw it. The collection is already pretty fantastic, and upon my arrival at the college, I saw an entire table full of fossil treasures. The museum displays were also great. Thanks to Mace for a successful (if all too brief) visit!

Mace Brown with a new eomysticetid fossil, which J. Geisler has graciously invited me to study with him. This was a recent acquisition, and Mace was still gluing parts of the mandibles back together. What an awesome fossil!

I furiously scribbled down notes - I had two 3-hour periods where I was able to examine the fossils at the College of Charleston. A herpetocetine petrosal is shown.

Al Sanders (right) and Ewan discuss fossil cetaceans. This was the first time Ewan and Al had met face to face since the 2006 SVP meeting - its tough, New Zealand is pretty far from everything.

Some sort of a nasty agorophiid-like dolphin.

A partial odontocete skeleton.

A beautiful xenorophid skull on display. Look at those teeth!

Some kind of a sea turtle - identified as Procolpochelys.

A skull of the sea turtle Carolinachelys.

Coming up next - I found some more photos from Charleston, so there will be STILL more from the US trip (I was gone for a whole month, I might as well talk about it). Also: reviews of some recent research by M. Churchill, R. Racicot, G. Aguirre, and N. A. Smith, and more on Osedax and taphonomy.

Number of visits

About the Coastal Paleontologist

I'm a paleontologist and adjunct faculty at College of Charleston in South Carolina, with research interests in Cenozoic marine vertebrates with an emphasis on marine mammals (whales, dolphins, pinnipeds, otters, sea cows, and others), but I willingly entertain brief distractions into the worlds of marine birds, sharks, and fish. My M.S. (2011, MSU-Bozeman) focused on marine vertebrate taphonomy whilst my Ph.D. (2015, U. Otago, NZ) focused on Oligocene baleen whales from New Zealand. Current research is concerned with fossil cetaceans from South Carolina including Oligocene eomysticetids, toothed mysticetes, and archaic dolphins.